It is well-known to provide an axle driving apparatus which houses therein a hydrostatic transmission (hereinafter referred to as an "HST") and a drive train for transmitting power from an output shaft of the HST to the axles. In such a case, it is common to provide a chamber in the housing in which the HST is located and a separate chamber in the housing in which the drive train and axles are located. The two chambers are partitioned from each other to prevent foreign objects, such as iron powder from the drive train, from entering into the HST chamber. Such a technique is disclosed in, for example, U.S. Pat. No. 5,440,951.
In that patent, the housing is partitioned into a first chamber for housing the HST and a second chamber for housing the drive train, such as a differential gear unit. A reservoir is formed in the upper portion of the housing to communicate with the first chamber and also with the second chamber. Oil fills the housing through an oil port provided in the reservoir.
In such construction, when the HST is working, the volume of oil in the first chamber is increased as the temperature of the oil in the first chamber increases and the increased oil enters into the reservoir. Conversely, when the volume of oil in the first chamber decreases, the oil returns into the first chamber from the reservoir. Air bubbles created in the oil in the second chamber by rotation of the drive train are collected in the reservoir. As a result, there is a possibility that foreign objects, such as iron powder, will flow together with the air bubbles and enter into the reservoir from the second chamber. In which case, oil contaminated by the foreign object in the reservoir enters from the reservoir into the first chamber. Foreign objects in the first chamber can enter into slidably rotatable portions of the HST, which can promote wear of the HST. Although the foreign object entering the first chamber is removed by an oil filter before being supplied as operating oil to the HST, the oil filter is quickly clogged and as a result, must be changed more often. Because it is very troublesome to change the oil filter, it is desired to have the period between oil filter changes as long a possible.